1. Field of the Invention
This invention relates to a data transmission system and particularly to a facsimile transmission system in which a receiver can send a request for retransmission of data in the form of a block upon detection of an error to the transmitter.
2. Description of the Prior Art
As well known in the art, in a facsimile communication system, in a protocol procedure prior to transmission of messages or image data, the transmission speed or rate of MODEM is selected to carry out MODEM training, and if successful, transmission of image data at the selected transmission speed is carried out. However, even if the MODEM training has been carried out successfully, it does not insure the transmission of image data in an excellent condition at all times. If the condition of transmission line becomes poor during a transmission operation, image data may be partly lost during transmission thereby hindering to transmit image data in a complete form to a receiver.
In order to cope with the above-described inconveniences, the ARQ method has been applied to a facsimile transmission system. In this case, image data is divided into blocks, each containing a predetermined number of bits, prior to transmission to a receiver. And, to each block data is added a start flag, stop flag, command ( including block number ) and CRC code so that the image data is formulated into a predetermined frame. Thus, the image data is transmitted from a transmitter to a receiver block by block. At the receiver, each block data is examined upon receipt and if it is found that there is abnormality in CRC code, the block number differs from the one expected at the receiver, and/or the flag is destroyed, then the receiver automatically sends a signal for requesting retransmission of block data to the transmitter.
In such a scheme, the same image data is repetitively transmitted from the transmitter to the receiver whenever malfunction occurs during transmission. Thus, it is insured that the image data received at the receiver is always normal without loss and alterations, thereby allowing the receiver to receive an image which is faithful to an original image transmitted from the transmitter at all times. In a transmission system equipped with such an ARQ function, the transmitter must receive a response signal from the receiver while the transmitter is transmitting the next following block data. Therefore, the above-described ARQ function becomes inoperative when data must be transmitted through a transmission route having a relatively large delay in transmission. For example, this happens when a submarine cable or communications satellite is involved. When a submarine cable is used, there is a transmission delay of approximately 50 milliseconds in one way; whereas, when a communications satellite is used, there is similarly a transmission delay of approximately 300 milliseconds. In such a case, since there is a relatively large delay in transmission, there is a chance that the transmitter fails to receive a response signal from the receiver as to the last preceding block data which has been just transmitted while the transmitter is transmitting the next following block data.